1. Field of the Invention
The present invention relates to an electrochromic (herein after simply referred to as “EC”) element, comprising an oxidative coloration layer composed of an nickel oxide and an electrolyte layer composed of a solid electrolyte. More, specifically, the invention relates to an EC element whose deterioration of characteristics due to repeated cycles of coloration and discoloration can be suppressed.
2. Description of Related Arts
An EC element is an element which utilizes an EC phenomenon to reversibly carry out coloration and discoloration, and the EC element has been used in an anti-glare mirror, a dimming glass, a display elements and the like. FIG. 2 shows an example of a laminated cross-section of a conventional EC element. In this element, the whole of the element is made transparent. On a transparent glass substrate 10, an ITO (Indium Tin Oxide) transparent electrode film 12 making up a lower electrode film, a mixed film 14 comprising an iridium oxide and a tin oxide making up an oxidative coloration layer and a tantalum oxide film 16 making up a solid electrolyte film, a tungsten oxide film 20 making up a reductive coloring film and an ITO transparent electrode film 20 making up an upper electrode film are formed thereon on this order. A parting line 22 is previously formed on one edge of the lower ITO transparent electrode film 12 by a laser etching process, so that an area 12a of the edge portion is electrically partitioned. One edge of the upper ITO transparent electrode film 20 is electrically connected to the separated area 12a of the lower ITO transparent electrode transparent film 12. Clip electrodes 24 and 26 for tapping an electrode are provided on both edge of the substrate 10. The clip electrode 24 is electrically connected to the lower ITO transparent electrode film 12, and the clip electrode 26 is electrically connected to the upper ITO transparent electrode film 20. A transparent sealing resin 28 is applied onto the upper ITO transparent electrode film 20, whereon a transparent glass serving as a transparent sealing member 30 is adhered to seal the whole of the laminated films. According to the configuration just mentioned, taking the clip electrode 24 as a positive pole and the clip electrode as negative pole and applying a voltage, both the oxidative coloration layer 14 and the reductive coloration layer 18 are colored. On the other hand, applying a reverse voltage (or making a shortage circuit), both coloration layers 14 and 18 are discolored.
Both the mixed film 14 comprising an iridium oxide and a tin oxide, and the tungsten oxide film 18 in the conventional EC element are blue colored, and at the time of coloring, the whole of the element is blue colored. For this reason, the conventional EC element cannot be used for an application in which a color should be seriously taken (i.e., an application where a hue of an image obtained through the element should not be changed). For example, a presently existing digital camera has a built-in ND (natural density) filter, and is configured so that when an amount of an incident light is large, the ND filter is driven by a motor to move toward a front side of an imaging element such as CCD (charge coupled device) to be darkened. If the ND element could be substituted by an EC elements which can darken, then the driving by a motor is not required. This can miniaturize a digital camera and can save an electric power. However, since the conventional EC element colors blue, which leads to bluish image being taken, and which makes it difficult to control white balance, the conventional EC element cannot be used instead of the ND filter.
In light of the above situations, we have suggested an EC element which realizes a gray color at the time of the coloration as Japanese Patent Application Number 2002-251856, which has not yet been published. Referring to drawings, this technique will be described. FIG. 3 shows an EC element disclosed therein. The parts common to those of FIG. 2 are assigned to the same symbols. An ITO transparent electrode film 12 making up a lower electrode film is formed on a transparent glass substrate 10, a nickel oxide film 32 making up an oxidative coloration layer, a tantalum oxide film 16 making up a solid electrolyte layer, a mixed film 34 comprising tungsten oxide and titanium oxide making up a reductive coloration layer, an ITO transparent electrode film 20 making up an upper electrode film are formed on this order. A parting line 22 is previously formed on one edge of the lower ITO transparent electrode film 12 by a laser etching process, so that an area 12a of the edge portion is electrically partitioned. One edge of the upper ITO transparent electrode film 20 is electrically connected to the separated area 12a of the lower ITO transparent electrode film 12. Clip electrodes 24 and 26 for tapping an electrode are provided on both edge of the substrate 10. The clip electrode 24 is electrically connected to the lower ITO transparent electrode film 12, and the clip electrode 26 is electrically connected to the upper ITO transparent electrode film 20. A transparent liquid sealing resin 28 is applied onto the upper ITO transparent electrode film 20, whereon a transparent glass serving as a transparent sealing member 30 is cured to seal the whole of the laminated films. According to the configuration just mentioned, the EC layer shown in FIG. 3 is made transparent in the thickness direction thereof.
According to the configuration shown in FIG. 3, taking the clip electrode 24 as a positive pole and the clip electrode as negative pole and applying a voltage, both the oxidative coloration layer 32 and the reductive coloration layer 34 are colored. On the other hand, applying a reverse voltage (or making a shortage circuit), both coloration layers 32 and 34 are discolored. The chemical reactions at the time of the coloration and discoloration are, for example, represented as follows:
[Coloration][Discoloration]Reductive Coloration Layer   [Solid Electrolyte Layer]  [Oxidative Coloration Layer]   
The productions stages for producing the EC element shown in FIG. 3 will be described as follows:    (1) A transparent glass substrate 10 on which an ITO transparent electrode film 12 is formed is prepared, and it is cut into a desired shape. Alternatively, a transparent glass substrate 10 having being cut into a desired shape is prepared, and an ITO transparent electrode film 12 is formed thereon.    (2) A parting line 22 is formed by laser-etching the transparent glass substrate 10.    (3) The substrate 10 is accommodated within a vacuum chamber of a vacuum deposition apparatus, a nickel oxide film 32 is formed on the substrate 10 by a vacuum deposition process utilizing NiO or Ni as a depositing material (starting material) When Ni is used as the depositing material, the deposited Ni is compounded to oxygen within the vacuum chamber to form a nickel oxide.    (4) Subsequently, a tantalum oxide film 16 is formed by a vacuum deposition process (precisely, an ion-plating process in which a high frequency is applied) utilizing Ta2O5 as a depositing material.    (5) Subsequently, by a two-element vacuum deposition process utilizing WO3 and TiO2, respectively, a mixed film 34 comprising tungsten oxide and titanium oxide is formed. An example of the arrangement of elements within the vacuum deposition apparatus at the time of carrying out this two-element vacuum deposition is shown in FIG. 4. A plurality of substrates 10 onto which the tantalum oxide 16 is formed are held in the state each surface subjected to the film formation is faced downwardly on a lower surface of a rotating substrate holder 11. Crucibles 13 and 15 are provided under the substrate holder 11. WO3 is accommodated within the crucible 13 as a depositing material 17. TiO2 is accommodated within the crucible 15 as a depositing material 19. Electron beams 21 and 23 are applied to the depositing materials 17 and 19, respectively to heat and volatilize the depositing materials 17 and 19. The volatilized depositing materials 17 and 19 go upwardly, are mixed, and then deposited on the substrate 10 to thereby form the mixed film 34 comprising tungsten oxide and titanium oxide. In inclined upper directions of the crucibles 13 and 15, quartz oscillators 25, and 27 are placed respectively. By a change in the frequency of the quartz oscillator 25, the deposition rate of the depositing material 17 is monitored. By a change in the frequency of the quartz oscillator 27, the deposition rate of the depositing material 19 is monitored. By controlling the outputs of the electron beams 21 and 23 depending upon the deposition rates of the depositing materials 17 and 19, which are monitored, a mixing ratio of tungsten oxide to titanium oxide in the mixed film 34 in a predetermined value. The series of the film formation stages (3) to (5) can be continuously carried out by switching the deposition sources and film formation conditions step by step while holding the substrate 10 on the substrate holder 11 without taking the substrate 10 out of the chamber.    (6) For a time, the substrate 10 is taken out of the vacuum chamber, a mask pattern is changed, and then the substrate 10 is again accommodated within the vacuum chamber, after which an upper ITO transparent electrode film 20 is formed by a vacuum deposition process (precisely, an ion-plating process in which a high frequency is applied) utilizing ITO as a depositing material. Amongst the film production processes (3) to (6), the formation of the tantalum oxide film 16 is carried out by an ion-plating process in which a high frequency of 600 W is applied in process (4), while the formation of the upper ITO transparent electrode film 20 is carried out by an ion-plating process in which a high frequency of 400 W is applied in process (6). The stages (3) and (4) are carried out without application of a high frequency. It is noted that the formation of the nickel oxide film 32 is carried out by an ion-plating process in which a high frequency is carried out, an originally colored nickel oxide film is formed. However, after the completion of the EC element, when a discoloration voltage is applied, the film is perfectly discolored and possessed characteristics similar to those produced without application of any high frequency. This indicates that the formation of the nickel oxide film 32 in stage (3) can be carried out with application of a high frequency.    (7) After the completion of film formation stages, the substrate 10 is taken out of the vacuum chamber, and clip electrodes 24 and 26 are provided.    (8) A transparent liquid sealing resin 28 is applied and a transparent glass as a sealing member 30 is adhered, and then the liquid sealing resin 28 is cured to complete the EC element.
The characteristics of the EC element measured, which is shown in FIG. 3, produced as described above will be described. In the measurement, approximately 4 cm square sample is used. The thickness of ITO transparent electrode film 12 is approximately 250 nm, that of the nickel oxide film 32 is approximately 100 nm, that of tantalum oxide film 16 is approximately 600 nm, a mixed film 34 comprising tungsten oxide and titanium oxide is approximately 500 nm, and that of ITO transparent electrode film 12 is approximately 250 nm. The film formation conditions of the nickel oxide film 32, the tantalum oxide film 16, a mixed film 34 comprising tungsten oxide and titanium oxide are as follows: The nickel oxide film 32 is formed by utilizing NiO having a purity of not less than 99.9% as a depositing material at a substrate temperature of 120° C. under an oxygen pressure of 3×10−4 Torr at a film formation rate of 0.5 nm/second. The tantalum oxide film 16 is formed by utilizing Ta2O5 having a purity of not less than 99.9% as a depositing material at a substrate temperature of 120° C. under an oxygen pressure of 3×10−4 Torr at a film formation rate of 0.67 nm/second. The mixed layer 34 comprising tungsten oxide and titanium oxide is formed by utilizing WO3 and TiO2 each having a purity of not less than 99.9% as depositing materials at a substrate temperature of 120° C. under an oxygen pressure of 1.6×10−4 Torr. The ratio of the deposition rate of WO3 to that of TiO2 is set depending upon the mixing ratio of the tungsten oxide and titanium oxide intended in the mixed film 34. For example, when the proportion of titanium atom (atomic %) in the mixed film 34 comprising a tungsten oxide and a titanium oxide relative to the total atomic number of tungsten atoms and titanium atoms is W:Ti=72:28, the ratio of volatilizing rates of WO3 to TiO2 (the ratio of increasing film thicknesses measured by the quarts oscillators 25 and 27) is set to be approximately 3:2.
The resulting nickel oxide film 32 is crystal (polycrystal). The resulting mixed film 34 comprising a tungsten oxide and a titanium oxide is amorphous. There is a possibility that in the course of depositing NiO, part of NiO is changed to nickel oxides [Ni(OH)2, Ni2O3, NiOOH and the like], but in any case, the NiO may be considered to be the main component in the nickel oxide film 32. Similarly, although there is a possibility that part of WO3 is chaned to other tungsten oxides and part of TiO2 is changed to other titanium oxides in the course of the two-element deposition stage, in any case, the mixed film 34 comprising a tungsten oxide and a titanium oxide may be considered to comprise a mixture of WO3 to TiO2 as main components. It is considered that the nickel oxide film 32 may exhibit an EC phenomenon even if it is in a crystalline, fine-crystalline or amorphous state. The mixed film 34 comprising a tungsten oxide and a titanium oxide is considered to be desirably amorphous, since efficiencies of coloration and discoloration are decreased when it is crystallized.
FIG. 5 shows spectral transmittance characteristics in the case where the mixing ratio of the tungsten oxide and the titanium oxide in the reductive coloration layer 34 in the EC element shown in FIG. 3 is varied. The value for the proportion of Ti (atomic %) is shown as the proportion of titanium atom (atomic %) in the mixed film 34 comprising a tungsten oxide and a titanium oxide relative to the total atomic number of tungsten atoms and titanium atoms determined by photoelectron spectroscopy (ESCA). In the measurement of FIG. 5, at the time of coloration, the applied voltage is 20 V, and the characteristics after the application for 90 seconds are measured. At the time of discoloration, the discoloration voltage is −2.0 V, and the characteristics after the application for 90 seconds are measured. According to FIG. 5, it has been proven that the spectral characteristics, particularly those at a long wavelength side are changed depending upon the mixing ratio of the tungsten oxide to the titanium oxide; as TiO2 is lower, the level at a long wavelength side is down, exhibiting a bluish color; and as TiO2 becomes higher, the level at a long wavelength side is up, decreasing a bluish color. Also, when the proportion of titanium atom is within the range of from 5 to 40 atomic % (i.e., the atomic ratio of W to Ti=95:5 to 60:40), the mixed layer 34 had a gray color. Particularly, when the proportion of titanium atom is within the rage of from 20 to 3-atomic % (i.e., the atomic ratio of W to Ti=80:20 to 70:30), the spectral characteristics at the visible region (400-80 nm) becomes nearly flat, a substantially pure gray color is obtained. It is noted that there is little differences in the spectral characteristics depending upon the mixing ratio of TiO2 at the time of the discoloration. In this case, transmittances at a human's view peak position (wavelength at 550 nm) are 80% or more (average transmittance at the total of the visible wavelength region is approximately 80%), obtaining colorless and substantially transparent color.
Next, this EC element will be described by referring to FIG. 6. FIG. 6 shows spectral transmittance characteristics in the case where the coloration voltage is varied and the discoloration voltage is applied in the EC element shown in FIG. 3. In the measurement in FIG. 6, the atomic proportion of titanium contained in the mixed film 34 comprising a tungsten oxide and a titanium oxide is 28 atomic % (atomic ratio of W:Ti=72:28). At the time of coloration, the characteristics are measured after a coloration voltage is applied for 90 seconds. At the time of discoloration, the characteristics are measured after a discoloration voltage, which is −1.5 V, is applied for 90 seconds. According to FIG. 6, it has been proven that at the time of the coloration, the transmittance becomes lower as the coloration voltage is higher, and the characteristics become flat at the visible region, lowering a bluish color. Furthermore, when the coloration voltage becomes higher than 1.75 V, the EC element is colored in a concentrated manner. Particularly, at the coloration voltage not less than 2 V, the transmittance is decreased near 10% or less over substantially entire visible region, the characteristics are much more flatten, obtaining substantially pure gray color. Consequently, the peak value of the applied voltage at the time of the coloration is preferably not less than 1.75V, more preferably not less than 2.0 V and not more than voltage resistance. The voltage at the time of the discoloration is also relatively flat at the visible region, and a colorless and substantially transparent color is obtained. The transmittance at the time of the discoloration is obtained 80% or more at the human's visible peak region.
According our experiments thereafter, it has been understood that the peak value of the applied voltage at the time of the coloration is desirably not more than 3 V. Specifically, at the applied voltage of not less than 3 V, the durability against repeatedly carrying out coloration and discoloration cycles is decreased. The example described above is the case of a transmissive element where ITO transparent electrode films are used as both electrodes (an EC element made transparent in the thickness direction thereof). In contrast, in the case of an reflecting element (EC element configured as a mirror) where an ITO electrode is used as one electrode and a reflecting film also serving as an electrode is used as the other electrode, the peak value of the applied voltage at the time of the coloration is desirably lower than that in the case of the transmissive electrode. Specifically, in the case of the reflecting element having a reflecting film also serving as an electrode formed, when the applied voltage at the time of the coloration is +2 V, it is destroyed after several ten cycles of the coloration and discoloration. Although the reason has not yet been known, since the reflecting film also serving as an electrode (Al film) has a lower electric resistance in comparison with that of the ITO transparent electrode film, it is deduced as one cause that the voltage applied to the EC element itself becomes high. According to our experiments, in the case of the reflecting element utilizing the reflecting film also serving as an electrode (Al film), it has been proven that the peak value of the applied voltage at the time of the coloration is desirably not less than 1 V and not more than 1.8 V.
Next, this EC element will be described by referring to FIG. 7. FIG. 7 shows characteristics for response speed at a time of the coloration in the case where the coloration voltage is varied in the EC element. In the measurement in FIG. 7, the atomic proportion of titanium contained in the mixed film 34 comprising a tungsten oxide and a titanium oxide is 28 atomic %. The transmittances is measured at a human's view peak position (wavelength at 550 nm). According to FIG. 7, the characteristics for response speed depends upon the area of the element, and as the element area is lower, the response becomes quicker, while as the element area is higher, the response becomes slower. In the case of utilizing the EC element as an element for regulating camera's exposure, the element area may be a relatively small and, if the coloration voltage is not less than 1.75 V, the coloration is arrived to a concentrated color at relatively instance.
Next, differences between the EC element of FIG. 3 and the conventional EC element of FIG. 2 in spectral transmittance characteristics are shown in FIG. 8. In the measurement in FIG. 8, the atomic proportion of titanium contained in the mixed film 34 comprising a tungsten oxide and a titanium oxide is 28 atomic % The coloration voltage is 2.0 V for the EC element of FIG. 3 and 1.5 V for the conventional EC element of FIG. 2, and the applied period is 30 second for both EC elements. The discoloration voltage is −1.5 V and the applied period is 30 seconds for both EC elements. According to FIG. 8, for the transmitting color at the time of the coloration, the conventional EC element of FIG. 2 is strongly bluish, while the EC element of FIG. 3 has flat characteristics at the visible region, obtaining a gray color. As for the transmitting color at the time of the discoloration, while the conventional EC element of FIG. 2, which has a high level around yellow color, becomes a yellowish color, the EC element of FIG. 3 becomes colorless and substantially transparent. The EC element of FIG. 3 has a transmittance at the coloration of approximately 20% over a substantially entire region, and a transmittance at the discoloration of approximately 80% at the peak position of the human visibility. The value of the transmittance, which is 80%, is high in comparison with that of the conventional EC element of FIG. 2, and it looks a high transparency in comparison with the conventional EC element of FIG. 2.
It is noted that no gray color can be obtained at the time of the coloration, only when the material of the oxidative coloration layer 14 is changed to the nickel oxide (comprising NiO as a main component) in the conventional configuration shown in FIG. 2. Also, no gray color can be obtained at the time of the coloration, only when the material of the reductive coloration layer 18 is changed to a mixture of tungsten oxide with titanium oxide (comprising WO3 and TiO2 as main components) A gray color at the time of the coloration can obtained for the first time when the material of the oxidative coloration layer 14 is changed to the nickel oxide (comprising NiO as a main component) and the material of the reductive coloration layer 18 is changed to a mixture of tungsten oxide with titanium oxide (comprising WO3 and TiO2 as main components).
As described above, the EC element of FIG. 3 which gives a transparent color at the time of the coloration and which gives a colorless and substantially transparent color at the time of the discoloration is suitable for the application where the change in the hue in the resulting image obtained-through the EC element is not desired. FIG. 9 schematically shows an example of the arrangement of main portions of an optic system in the case where the EC element of FIG. 3 is used in a digital camera (steel camera, video camera or such). On an optical axis from a lens 36 to an imaging element 42 such as CCD, a mechanical diaphragm 38 driven by a motor and an ND filter comprising the EC element shown in FIG. 3 are arranged on this order. The ND filter 40 comprising an EC element is arranged in a fixed manner. When a luminescence of a subject measured by a photometric element, which is separately provided, is within a prescribed value, the NF filter 40 comprising an EC element is discolored. The average transmittance of a visible light possessed by the ND filter 40 comprising an EC element is approximately 80% at the time of the discoloration and, thus, there is no case for lacking exposure. Also, since the transmitting color is colorless and substantially transparent, the image taken does not become bluish, and a white balance can easily be controlled. When the luminescence of a subject becomes larger than a prescribed value and, when no exposure cannot be adequately adjust at F 38, a prescribed coloration voltage (for example, 2.0 V) is applied to the ND filter comprising an EC element to make the ND filter 40 in a coloration state, and to be darkened. At the time of the coloration, the average transmittance of a visible light possessed by the ND filter 40 comprising an EC element is approximately 20%, which can sufficiently decrease a light. Since the transmitting color is gray, good color reproductivity can be obtained, and the image taken does not become bluish. Thereafter, if the luminance of the subject is decreased to be not higher than the prescribed value, a prescribed discoloration voltage is applied to the ND filter 40 comprising an EC element (or both electrodes are shorted) to be in a discoloration state. As described above, by substituting the conventional motor-driven ND filter with the ND filter 40 comprising an EC element, a mechanical configuration becomes simple, making it possible to miniaturize a camera, enhancing a design, and reducing a weight. Also, since driving by a motor is not required, an electric power can be saved. Also, the ND filter 40 comprising an EC element requires no attachment and detachment, operation of attachment and detachment can be avoided. The ND filter 40 comprising an EC element can also be accommodated within a single lens reflex camera. Since the the ND filter 40 comprising an EC element is all solid type, even if it is destroyed, there is no problem in terns of leakage of a liquid or such.
Whereas the example described above is that the adjustment of the coloring amount possessed by the ND filter 40 comprising an EC element is one stage (ON/OFF) switching manner, it may be two or more switching manner or no switching manner by adjusting the coloration voltage to adjust the coloring amount in two or more stages or no stage (the peak value of the coloration voltage is se to be, e.g., 1.75 V or more, more preferably 2 V or more and 3V or less). In the case of the configuration just mentioned, the ND filter 40 comprising an EC element can also serve as diaphragm and the mechanical diaphragm 38 may be omitted, making the mechanical configuration much more simple.
The EC element of FIG. 3 may not only be used in a digital camera, but also in an exposure adjusting element for a film type camera. Also, the EC element of FIG. 3 may be used in sunglasses, a dimmer glass, sunroof and the like.
According our studies thereafter, it has been proven that the EC element disclosed in Japanese Patent Application No. 2002-251856 deteriorates the response in a relatively early stage when the cycles of coloration and discoloration are repeated. Although it has not yet been known that what type of chemical reaction leads to the deterioration in the characteristics due to the repeated cycles of the coloration and discoloration, it may be considered that some types of deterioration occur on an interface between the nickel oxide and the solid electrolyte or between the nickel oxide and the solid electrolyte.
The present invention is made in light of the above situations, and an object of the present invention is to enhance durability of the EC element comprising the oxidative coloration layer composed of an nickel oxide and the electrolyte composed of a solid electrolyte against repeated cycles of coloration and discoloration.